1. Technical Field
The present invention relates to a synchronous detection circuit, a sensing circuit, a physical quantity measuring device, a gyro sensor, an electronic apparatus, and the like.
2. Related Art
WO2005-068939 is an example of related art. It discloses a sensing circuit that amplifies a charge (a physical quantity signal) from a sensor, removes an unnecessary signal (a signal whose phase is shifted 90 degrees with respect to a regular physical quantity signal) by synchronous detection, smoothes the signal after the synchronous detection with a smoothing circuit (a low pass filter) to convert it to a direct current voltage, and amplifies the direct current voltage signal, for example.
In a no-input state, a level of a sensed output (a direct current voltage) of the sensing circuit should match with the direct current reference voltage (a direct current bias voltage) in a signal path. However, the level of the sensed output is actually different from the direct current reference voltage. A voltage difference between the actual sensed output and the direct current reference voltage is called an offset voltage. That is, the actual sensed output is a voltage that the direct current reference voltage is superposed on the offset voltage. Various reasons cause the offset voltage. For example, an unnecessary leakage signal, or a leakage current of a transistor in the non-put state causes the offset voltage.
The offset voltage results in sensing a signal by the sensing circuit even there is no signal from the sensor, thereby a measurement error occurs. Especially, when an input signal (the charge) is minute and a frequency is low (e.g., approximately several hundred Hz), reducing the offset voltage becomes necessary to improve sensing accuracy.
Conventionally, to cancel the offset voltage, a method is employed in which an offset cancel voltage (a voltage that has the same potential difference with respect to a direct current reference voltage as the offset voltage and its polarity is different) is superposed on (added to) a signal path of a sensing circuit.
In the technique disclosed in the example of related art, a method is employed in which an offset cancel voltage is superposed on an input signal (a detection signal to be synchronized) of the synchronous detection circuit so as to synchronously detect the detection signal to be synchronized on which the offset cancel voltage is superposed.
In the technique disclosed in the example of related art, compensating variation of the direct current reference voltage which depends on the ambient temperature in a signal path is not considered. If the direct current reference voltage in the signal path varies in accordance with the ambient temperature, a level of a direct current voltage after the synchronous detection varies, and a sensing error occurs. Therefore, compensating the ambient temperature dependency of the direct current reference voltage of the signal path enables sensing accuracy to be improved.
In order to cancel temperature dependency of the direct current reference voltage, a temperature compensation voltage having an opposite characteristic from a temperature offset of the direct current reference voltage is superposed on a signal (an alternating current signal) of a detection object. However, it is necessary to choose a position on which the temperature compensation voltage to be superposed by taking a 1/f noise and the like into consideration so that the noise is most reduced.
Further, the offset compensation voltage and the temperature compensation voltage are independent from each other (uncorrelated parameter). Therefore, it is necessary to generate the above two voltages that are uncorrelated and independent from each other, and the voltages are independently superposed (added) without a correlation.